[ecash-herald] Show address for self-send txs when there is only 1 address

Summary: Now that we have an address directory, it can be interesting to know when some addresses are sending to themselves

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14231